Polish the UART example.

rp2040-hal/examples/uart.rs

@@ -1,32 +1,64 @@
-//! Print an incrementing number to UART0 in a loop
+//! # UART Example
+//!
+//! This application demonstrates how to use the UART Driver to talk to a serial
+//! connection.
+//!
+//! It may need to be adapted to your particular board layout and/or pin
+//! assignment.
+//!
+//! See the `Cargo.toml` file for Copyright and licence details.
+
 #![no_std]
 #![no_main]
 
-use core::fmt::Write;
+// The macro for our start-up function
 use cortex_m_rt::entry;
-use hal::clocks::init_clocks_and_plls;
-use hal::gpio::{self, Pins};
-use hal::pac;
-use hal::sio::Sio;
-use hal::uart::UartPeripheral;
-use hal::watchdog::Watchdog;
+
+// Ensure we halt the program on panic (if we don't mention this crate it won't
+// be linked)
 use panic_halt as _;
+
+// Alias for our HAL crate
 use rp2040_hal as hal;
 
+// A shorter alias for the Peripheral Access Crate, which provides low-level
+// register access
+use hal::pac;
+
+// A GPIO trait we need
+use embedded_hal::digital::v2::OutputPin;
+use embedded_time::fixed_point::FixedPoint;
+use rp2040_hal::clocks::Clock;
+
+/// The linker will place this boot block at the start of our program image. We
+// need this to help the ROM bootloader get our code up and running.
 #[link_section = ".boot2"]
 #[used]
 pub static BOOT2: [u8; 256] = rp2040_boot2::BOOT_LOADER;
 
+/// External high-speed crystal on the Raspberry Pi Pico board is 12 MHz. Adjust
+/// if your board has a different frequency
+const XTAL_FREQ_HZ: u32 = 12_000_000u32;
+
+/// Entry point to our bare-metal application.
+///
+/// The `#[entry]` macro ensures the Cortex-M start-up code calls this function
+/// as soon as all global variables are initialised.
+///
+/// The function configures the RP2040 peripherals, then writes to the UART in
+/// an inifinite loop.
 #[entry]
 fn main() -> ! {
+    // Grab our singleton objects
     let mut pac = pac::Peripherals::take().unwrap();
-    let mut watchdog = Watchdog::new(pac.WATCHDOG);
-    let sio = Sio::new(pac.SIO);
+    let core = pac::CorePeripherals::take().unwrap();
 
-    // External high-speed crystal on the pico board is 12Mhz
-    let external_xtal_freq_hz = 12_000_000u32;
-    let clocks = init_clocks_and_plls(
-        external_xtal_freq_hz,
+    // Set up the watchdog driver - needed by the clock setup code
+    let mut watchdog = hal::watchdog::Watchdog::new(pac.WATCHDOG);
+
+    // Configure the clocks
+    let clocks = hal::clocks::init_clocks_and_plls(
+        XTAL_FREQ_HZ,
         pac.XOSC,
         pac.CLOCKS,
         pac.PLL_SYS,
@@ -37,14 +69,20 @@ fn main() -> ! {
     .ok()
     .unwrap();
 
-    let pins = Pins::new(
+    let mut delay = cortex_m::delay::Delay::new(core.SYST, clocks.system_clock.freq().integer());
+
+    // The single-cycle I/O block controls our GPIO pins
+    let sio = hal::sio::Sio::new(pac.SIO);
+
+    // Set the pins to their default state
+    let pins = hal::gpio::Pins::new(
         pac.IO_BANK0,
         pac.PADS_BANK0,
         sio.gpio_bank0,
         &mut pac.RESETS,
     );
 
-    let mut uart = UartPeripheral::<_, _>::enable(
+    let mut uart = hal::uart::UartPeripheral::<_, _>::enable(
         pac.UART0,
         &mut pac.RESETS,
         hal::uart::common_configs::_9600_8_N_1,
@@ -52,8 +90,9 @@ fn main() -> ! {
     )
     .unwrap();
 
-    // UART TX (characters sent from pico) on pin 1 (GPIO0) and RX (on pin 2 (GPIO1)
+    // UART TX (characters sent from RP2040) on pin 1 (GPIO0)
     let _tx_pin = pins.gpio0.into_mode::<gpio::FunctionUart>();
+    // UART RX (characters reveived by RP2040) on pin 2 (GPIO1)
     let _rx_pin = pins.gpio1.into_mode::<gpio::FunctionUart>();
 
     uart.write_full_blocking(b"UART example\r\n");